Also co-routines with it's under the hood management of coroutine stack probably would've complicated GC support even more...
At the time the two main customers of having a C++ GC would be Unreal C++ and C++/CLI, the design that landed on the standard serves neither of them, thus no one adopted it.
Having written a bunch of more or less complete GC based runtimes, the boundary between C++ code and runtime code is often painful mostly due to GC tracking and reading the article I can only concur with what they're doing with Oilpan for V8/Chrome.
Having had a way in C++ to interrogate the current stack and then trace objects (via C++26 reflections) would've sufficed and probably alleviated the need for a separate GC.
Which is why I became kind of vocal regarding existing practice, or at least preview for community feedback, as there are already several examples of stuff landing into the standard that were not properly baked to start with.
Wrote a bit more in the sibling comment to pjmpl.